Observer tracking autostereoscopic 3D displays are disclosed, for instance, in EP 0 656 555, EP 0 726 482, EP 0 404 289 and in "Eye-Position Tracking Stereoscopic Display Using Image Shifting Optics", H. Imai et al, SPIE Vol. 2653, pp 49-55, February 1996. In such displays of the autostereoscopic 3D type, viewing zones are formed such that, when an observer is disposed so that left and right eyes are in adjacent viewing zones, a 3D image is perceptible. In order to allow the observer more freedom of movement while maintaining the 3D effect, the position of the observer may be measured, for instance by the technique disclosed in British Patent Application No. 9616190.6, and this information may be used to control the display so that the viewing zones move with or track the observer. In the case of EP 0 726 482, observer tracking is performed by changing the two dimensional (2D) images provided in the viewing zones as the observer moves laterally with respect to the display.
"Stereoscopic liquid crystal display I (General description)", T. Hattori et al, Pmroc. SPIE vol. 2177, pp 143-149, February 1994 discloses an autostereoscopic 3D display in which 2D images displayed by spatial light modulators are illuminated by a 2D display which acts as a backlight and co-operates with a converging optical system to direct the different 2D views into the different viewing zones. An observer is illuminated by an infrared source and monitored by an infrared video camera. The image from the video camera is effectively displayed by the 2D monochrome display so as to provide observer tracking by forming on the 2D display a bright patch which follows movement of the observer.
FIG. 1 of the accompanying drawings illustrates a mechanically tracked autostereoscopic 3D display comprising a backlight 1 which illuminates a spatial light modulator (SLM) in the form of a liquid crystal device (LCD) 2. A movable lenticular screen 3 is disposed between the observer and the LCD 2 and comprises a plurality of cylindrically converging lenticules such as 4. Each lenticule 4 is optically aligned with two columns of picture elements (pixels) such as 5 and 6. Alternate columns of pixels display vertical strips of a respective 2D image and the lenticules 4 direct light from the backlight passing through the columns 5 and 6 into two viewing zones 7 and 8 for the left and right eyes of an observer.
An observer tracking sensor (not shown) detects the position of the observer and the lenticular screen 3 is moved laterally with respect to the LCD 2 in response to the observer measured position so that the left and right eyes of the observer are maintained in the viewing zones 7 and 8, respectively.
The display shown in FIG. 2 of the accompanying drawings differs from that shown in FIG. 1 in that the lenticular screen is replaced by a parallax barrier 3. The parallax barrier 3 comprises a plurality of parallel evenly spaced vertical slits such as 4 which form the viewing zones 7 and 8 in essentially the same way as the lenticules of the display shown in FIG. 1. The parallax barrier 3 is movable laterally with respect to the LCD 2 as indicated by arrows 9 and 10 so as to track lateral movement of the observer as indicated by an arrow 11.
As shown in FIG. 3 of the accompanying drawings, the moving optic 3, which may be the lenticular screen or the parallax barrier shown in FIGS. 1 and 2, is mechanically connected to an electromechanical actuator 12, such as a voice coil stage. The stage 12 is controlled by a stage controller 13 in the form of a servo having positional feedback indicated at 14.
A tracking sensor 15 measures the position of the observer 16 and supplies the measurement signals to an arrangement 17 for converting the signals from the sensor 15 into a measurement of the position of the observer 16 relative to the display. The measured position is then compared with a display calibration 18 so as to determine the appropriate position of the moving optic 3 relative to the LCD 2. The calibration comprises information stored in a look-up table determined as a result of calibrating the display. The required movement of the moving optic 3 is determined at 19 and supplied to the stage controller 13. The moving optic 3 is thus moved so that the observer eyes remain in the viewing zones.
The tracking sensor 15 may comprise a video camera connected to an image processor for detecting the position of the head of the observer. However, such systems are expensive because of the cost of the camera and, in particular, the cost of the image processor.
Another known type of tracking sensor uses a magnetic position detector. However, such systems are prone to magnetic interference and require the user to wear a detector attached to the display by a cable.
Another type of tracking sensor relies on detecting the position of the head of the observer by means of reflection of infrared by the observer or by a retro-reflective spot, for instance stuck on the forehead of the observer. The reflected infrared radiation is imaged by a lens on to a position sensitive detector (PSD) as shown in FIG. 4 of the accompanying drawings. An infrared light emitting diode (LED) 20 emits infrared light which is supplied via an illumination lens 21 to illuminate a region in front of the display where an observer may be tracked. The infrared light reflected from the retro-reflecting dot or target 22 is collected by a collection lens 23 and is imaged through an infrared-passing filter 24 on the PSD 25. The PSD 25 may be of known type and supplies data indicating the position on the light-sensitive surface thereof of the "centre of gravity" of illumination "or centre of illumination".
These known arrangements suffer from various disadvantages. Firstly, for mechanically tracked systems, the alignment of the observer tracking sensor and the mechanics controlling the position of the parallax optic has to be highly accurate and robust since, otherwise, the display would lose calibration and the observer would start to lose the 3D image. This places severe requirements on the complexity and tolerance, and hence cost, of the display. Further, calibration data have to be generated during assembly of the display so as to account for differences in the alignment of the tracking sensor with respect to the remainder of the display. This substantially increases assembly cost. Further, the actual position of the actuator 12 may differ from the position indicated by the positional feedback 14, for instance because of backlash, inaccuracies in positioning and system lags.